A structure of a bead part in pneumatic radial tires for heavy load has so far been studied in various manners in order to satisfy a motion performance and a durability performance required to tires. Further, improvement in a high speed performance of tires has come to be required according to development in highways and speed-up of vehicles, and great force is exerted more and more on a rubber chafer disposed between a rim and a bead so that a rubber chafer provided with further durability is increasingly needed.
Performances required essentially to a rubber chafer include abrasion resistance to a rim, slipping resistance to a rim, an air sealing property, an air barrier property, exhausting resistance, crack resistance and the like, and since required performances residing in an antinomic relation are present among them, a rubber chafer has so far been constituted by selecting one or more kinds of rubber materials which can exert the above respective performances in moderation under mutual compromise.
However, pneumatic tires for heavy load which are provided with the above rubber chafer have involved the problems that they are liable to bring about abrasion to a rim, cracks and the like on a part brought into contact with a rim flange in a period extending from applying no load to applying a 200% load at a relatively early date and in addition thereto, have low weatherability so that they damage a visual quality of the tires and reduce durability thereof.
Then, known as tires which solve the foregoing problems of pneumatic tires for heavy load provided with a rubber chafer are, for example, pneumatic tires wherein the rubber chafer is prepared from a rubber composition in which 0 to 100 parts by weight of carbon black and 20 to 120 parts by weight of silica having a specific surface area by nitrogen adsorption (N2SA) of 4-0210 to 300 m2/g are compounded based on 100 parts by weight of a diene base rubber and a total amount of carbon black and silica is 50 to 130 parts and in which an organic silane compound is blended in an amount of 5 to 25% by weight based on the weight of silica and wherein increase in an elastic modulus for securing a motion performance is intended to be compatible with fractural resistance of a toe part and abrasion resistance of a rim flange (refer to, for example, patent document 1).
Further, tires in which an elastic modulus of a second stiffener rubber in 100% elongation is increased more than an elastic modulus of a chafer rubber in 100% elongation in order to enhance flexural rigidity of a bead part as much as possible are present among radial tires for aircrafts. In the above case, since the second stiffener rubber is too hard, the second stiffener rubber is likely to blow on a running condition of high heat build-up, and the second stiffener rubber is likely to be peeled off in running for a long time. Known are radial tires for aircrafts in which considering the above problems, the foregoing troubles in the bead part such as abrasion to a rim, blow and peeling are intended to be prevented by using for a side rubber a different kind of rubber from the chafer rubber and reducing an elastic modulus of a second stiffener rubber in 100% elongation less than an elastic modulus of a chafer rubber in 100% elongation (refer to, for example, patent document 2).
However, the pneumatic tires for heavy load described in patent documents 1 and 2 described above have durability and the like which have not so far been observed, but they are not yet satisfactory to performances required in recent years, and the existing situation is that pneumatic tires for heavy load in which a part of a rubber chafer brought into contact with a rim flange, particularly in a period extending from applying no load to applying a 200% load in a state of applying a suited air pressure is improved further in abrasion resistance to a rim, crack resistance and weatherability and which is further excellent in durability are desired to be developed.